Paper mills have for many years made extensive use, for the cleaning of paper making stock, of pressure screening apparatus embodying a perforated screening cylinder which defines screening and accepts chambers on the opposite sides thereof in a vertical housing, and wherein a rotor member operates in one of the chambers to keep the screening perforations open and free from solid material having a tendency to cling to the surface of the screening cylinder.
The assignee of this invention has manufactured and sold many such pressure screens in accordance with a series of U.S. patents, commencing with Staege U.S. Pat. No. 2,347,716, and followed by Martindale U.S. Pat. No. 2,835,173 and numerous other patents including Seifert U.S. Pat. Nos. 3,849,302 and 4,105,543. In operation, the stock or furnish is delivered to the screening chamber adjacent one end of the screening cylinder, and the material rejected by the screening cylinder is collated and discharged from the opposite end of the screening chamber.
In all of the vertically oriented commercial screens in accordance with any of the above patents, the primary direction of through flow is downward, with the stock entering the screen chamber from above, or in some cases centrally of the screening chamber when the direction of screening is from the outside to the inside of the screen member, so that any high specific gravity reject particles entrained with the stock to be screened will travel to a reject discharge chamber in the lower part of the screen, from which it is subsequently discharged. Necessarily, therefore, there is opportunity for such reject particles to damage the perforate screening cylinder as it travels through the screening chamber, especially with screens of the type wherein the screening chamber is on the inside of the screening cylinder, and wherein centrifugal force therefore will cause high specific gravity particles to travel along the screening surface.
In order to cope with this problem, it has been proposed to provide screening apparatus of the general type outlined above with a stock inlet chamber below the screening chamber, and to collect heavy reject particles from that inlet chamber so that it does not reach the screening chamber. Co-owned U.S. patents to Weber U.S. Pat. No. 4,166,028 and Martin et al U.S. Pat. No. 4,851,111 show such screening apparatus wherein the inlet chamber is below the screening chamber, and wherein a tangentially directed outer port for heavy reject particles lead from the bottom of the inlet chamber to a collection chamber which is periodically or intermittently opened to purge collected heavy reject particles therefrom.
This provision for removing heavy reject particles has not proved to be as successful as had been hoped, especially when the feed stock contains substantial quantities of particles in the form of pebbles and tramp metal as is often the case in pulp mills. The problem has been that substantial quantities of the heavy reject particles entering the inlet chamber are swept past the reject port therefor and continue to circulate around the lower portion of the outer wall of the inlet chamber. Further, since the rate at which this circulation occurs may be relatively high, the circulating heavy particles tend to abrade the wall of the inlet chamber.